Tribological arrangement

ABSTRACT

Arrangement comprising parts which rub against each other, in which arrangement the contacting surface of the one part comprises a polyketone and the contacting surface of the other part comprises a polymer.

FIELD OF THE INVENTION

The present invention relates to an arrangement comprising parts whichrub against each other.

BACKGROUND OF THE INVENTION

Such arrangement will hereinafter be called a tribological arrangement.Examples of tribological arrangements are sets of gears and bearings.Further examples are belt chains, plane and roller bearings, linearbearings, sleeve bearings, pulleys, sliding plates and other similardevices which have long been used to transmit, communicate, orfacilitate motion and power in mechanical devices.

In many cases, metal parts are used in tribological arrangements. Partshaving polymeric contacting surfaces form an attractive alternativeoffering one or more of the following advantages: greater shock andvibration dampening, reduced weight, enhanced corrosion protection,reduced running noise, decreased power and maintenance use, and morefreedom of component design. These advantages are especially importantin applications such as printers, copying machines and householdappliances, such as shaving apparatus and video apparatus. The polymersoften used in tribological arrangements are nylon and polyacetal.However, tribological arrangements made of such polymer show less goodwear, i.e. the amount of material worn off relative to the force appliedto the material. This means that the arrangements have an arc relativelyshort life time.

It would be desirable if the parts of the arrangement would be amenableto machining or processing. A further requirement is that the materialproperly holds a tolerance, is able to withstand the torsional stressesof start-up and shut-down, and from cyclical fatigue.

Problems can result in a failure of the tooth from excessive wear whichcan be compounded by plastic flow or creep due to thermal softening.Additionally, tooth bending fatigue, contact fatigue (pitting andspalling), thermal fatigue, tooth bending impact, tooth shear, toothshear, tooth clipping, case crushing, torsional shear, and stressruptures have similar impact. The role of material selection andpreparation thus clearly have much to do with the successful design of amotion and power transmission strategy. Low heat resistance, largethermal deformation, large shrinkage upon processing, and mediocremechanical strength have precluded the use of numerous thermoplastics,thermosets, and resins from serious consideration in demandingapplications.

While these considerations clearly apply to the structure andmanufacture of gears, it should also be borne in mind that almost anydevice used to transmit, communicate, or facilitate power and motionnecessarily involves similar concerns. For example, cams are generallyused to communicate motion and power by means of a connection between anedge (or a groove therein) or surface and a follower. In addition torepetitive/cyclic movement and imposition of force acting on both camand follower, these mechanisms are often designed to - incorporate largeaccelerations. Thus, the use of materials which cannot hold a tolerance(e.g., through loss of material) or which are not amenable to precisionprocessing or manufacture can easily result in wildly eccentric motionand, ultimately, failure. It is therefore important to employ materialswhich can be worked to precision, which function well when placed incommunication with each other, and which can withstand repetitive/cyclicmovement and impact.

The same can be said of bearings with rolling or sliding contact. Therepetitive and cyclic facilitation of motion inevitably raises concernsanalogous to those of gears and cams.

Indeed, one skilled in the art will readily appreciate that from amaterials perspective, an improvement in a means for transmitting powerand motion is generally applicable to all means of power and motiontransmission.

Rolling contact and sliding contact are manifested in most power andmotion gear applications. Rolling contact predominates in motion andpower transmission when such applications are between parallel shafts orbetween intersecting shafts. When non-parallel and non-inter- sectingshafts are employed, sliding contact predominates. Thus, materialsuseful in power and motion transmission between all shaft set-upsexhibit both good wear and good strength.

SUMMARY OF THE INVENTION

The present invention provides a tribological arrangement comprisingpolymer contacting surfaces and having a high wear resistance. Thesearch for such arrangement is hindered by the fact that it is difficultto predict the tribological properties of a certain combination ofpolymeric materials, even if both are known from different combinations.

The present invention relates to an arrangement comprising parts whichrub against each other, in which arrangement the contacting surface ofone part comprises a polyketone which is a linear alternating polymer ofcarbon monoxide and an olefinically unsaturated compound and thecontacting surface of another part comprises a polymer, preferably apolymer based on carbon, hydrogen and oxygen, which polymer is selectedfrom the group consisting of polyacetals and polyketones which arelinear alternating polymers of carbon monoxide and an olefinicallyunsaturated compound. Such arrangement shows especially good propertieswhen used at a relatively high torque. Furthermore, the presentinvention relates to a system for transmitting power and motioncomprising at least two means for transmitting power and motion whereinat least one means comprises a polyketone polymer and communicates powerand motion to another such means comprising a polyketone polymer. Thetransmission of motion and power includes the translation, communicationor facilitation of power and motion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows that polyketone gears endure torques for a much greaterperiod of time.

FIG. 2a shows the test run at torque of 4.3 N.m.

FIG. 2b shows the test run at torque of 11 N.m.

FIG. 3 shows the test run at torque 5.7 N.m.

DETAILED DESCRIPTION

Gears according to the present invention have been found to have toothstrength and wear resistance so that they are capable of bearing torquesfor power and motion transmission.

It has further been found that the low mold shrinkage that is attainablewhen such gears are, for example, injection molded results in excellenttooth quality. That is, there is very little error in single pitch,neighbouring pitch, accumulating pitch, etc. Thus, not only are spurgears successfully made according to this invention, but so too arecylindrical gears, helical gears, double helical gears, straight bevelgears, spiral bevel gears, zerol bevel gears, crossed helical gears,hypoid gears, worm gears, multithread worm gears, and other gears inwhich the intermeshing pitch circle is not on the same piane as thedirection in which the force is transmitted-or their pitch helix is notparallel to the axial direction of the cylinder.

Suitable means for transmitting motion and power include gears, belts,chain and sprocket assemblies, plane and roller bearings, linearbearings, sleeve bearings, pulleys, sliding plates and other likemechanisms. Preferred means are gears. Most preferred means are spurgears.

The systems of the present invention have been found capable oftransmitting power and motion up to the point of the mechanical failureof the means. Thus, for example, when the system comprises two or morespur gears, the gears will communicate motion and power until gear teethbreak. This is atypical of polymeric gear systems which typically failto communicate motion and power well before such breakage occurs.Rather, such systems of the prior art generally exhibit tooth wear untileach tooth of one gear slips past the teeth of the gear with which it isto communicate.

With the contacting surfaces of the parts is meant the surfaces whichrub against each other. As will be apparent, the advantage of thepresent invention is attained if the contacting surfaces are madeaccording to the invention. However, for ease of manufacture, it ispreferred that not only the surfaces of the parts will be of thesespecific polymers, but the part as a whole will be made of the polymer.In such case, the tribological arrangement comprises one part whichconsists of the polyketone while the other part consists of a polymer,preferably one based on carbon, hydrogen and oxygen. As will bediscussed hereinafter, the parts can optionally further containadditives. Fillers, extenders, lubricants, pigments, plastizers, andother polymeric materials can be added to the compositions to improve orotherwise alter the properties of the composition.-The amount ofadditive which can be present can be as high as 50% by weight.

A preferred tribological arrangement according to the present inventioncomprises a part having a surface comprising a polyketone and anotherpart having a surface comprising a polyacetal. The polyacetal can be ahomopolymer or a copolymer. A suitable polyacetal is polyoxymethylene,e.g. "POLYACETAL H", a polyacetal polymer commercially available from DuPont, or a copolymer containing a major amount of groups according tothe formula --CHR--O--and a minor amount of groups according to theformula --CH₂ --CH₂ --0, in which R is hydrogen or an alkyl group.

The polyketone used in the present invention is a linear alternatingpolymer of carbon monoxide and an olefinically unsaturated compound,i.e. a linear polymer containing equimolar quantities of the monomerscarbon monoxide and unsaturated compounds in which polymer substantiallyevery monomer unit which has been derived from an unsaturated compoundis positioned next to a monomer unit which has been derived from carbonmonoxide, and the other way round. Preferably, the polyketones arelinear alternating polymers of carbon monoxide and an olefinicallyunsaturated compound such as ethene, propene, butene, isobutene,amylene, butadiene, isoprene and/or vinyl compounds such as styreneand/or alphamethyl styrene. Preferably, the olefinically unsaturatedcompound is ethene, propene and/or 1-butene. A preferred polyketone isbuilt from carbon monoxide, ethene, and optionally one or more otherunsaturated hydrocarbons, such as hydrocarbons of from 3 to 20 carbonatoms, inclusive, preferably from 3 to 10 carbon atoms inclusive.Preferably, the other unsaturated hydrocarbon is propene and/or1-butene. A preferred polyketone has been described in EP-A-213 671.Attractive linear - alternating polyketones for use in the invention arelinear alternating polymers of carbon monoxide, ethylene and propylene.The propylene content should preferably be less than 7.0% by weight ofthe polyketone for a good heat distortion temperature, preferably from1.0 to 4.0 % by weight of the polyketone. Especially preferred for usein the present invention are polyketones having a limiting viscositynumber (LVN) measured in m-cresol at 60° C. of from 1.0 to 5.0,preferably from 1.3 to 4.0 dl/g, in particular from 2.1 to 3.0 dl/g.

Methods of making the polyketone are known in the art. Suitable methodshave been described in the following documents: EP-A-307 027, EP-A-181014, EP-A-121965, EP-A-391 579, EP-A-314 309 and European patentapplication no. 92203697.5. Polyketones obtainable by these processesare especially suitable for use in the present invention.

The polyketone can be present as such, or it can be reinforced by mixingit with mica and/or glass fibres. Mixtures which are especially suitablehave been described in EP-A-306116 and in EP-A-474309.

The polymers from which the contacting surfaces have been made canfurther have been mixed with internal lubricants such aspolytetrafluoroethylene, graphite, molybdenum disulfide, and variousoils to enhance wear resistance and to decrease frictional losses. Thepresence of an internal lubricant or an external lubricant, such as oilor grease, can further increase the wear resistance and reducefrictional losses between the contacting surfaces. A preferred internallubricant is a silicone oil. Useful silicone oils can be described aslinear chains of polydimethyl siloxane with viscosities ranging from1.000-300.000 centistokes. Silicone oils with high viscosities of from100,00w centistokes are preferred. Typically, silicone oil(s) arepresent in an amount of from 0.1-5 wt%, and preferably from 2-4 wt%based on weight of polyketone.

It has been found that the presence of suitable silicone oils means thatthe dynamic coefficient of friction (DCOF) is lowered, while thelimiting pressure velocity (LPV) is increased. During relative motion oftwo surfaces in contact the DCOF is the ratio of the resultingfrictional force to the applied normal force while holding the relativesurface velocity constant over time.

While holding the relative surface velocity constant between twospecimens in contact and increasing the applied normal force in astepwise manner in time, the LPV is the multiplicative product of thenormal pressure and surface velocity at the step just prior tocatastrophic material failure due to thermal softening.

A further preferred internal lubricant is a combination of fluorinatedhydrocarbons and silicone oils. Fluorinated hydrocarbons(fluoropolymers) useful in this application typically have a meltingtemperature at least 5-10 degrees above 278° C. Examples of suchfluoropolymers include perfluoroalkoxy resin (PFA),ethylene-tetrafluoroethylene (ETFE), fluorinated ethylene propylene(FEP) and polytetrafluoroethylene (PTFE). PTFE is preferred. Thefluoropolymers are generally present in an amount of from 1-20 wt%, andpreferably from 5-15 wt% based on total of polyketone.

Useful silicone oils are linear chains of polydimethyl siloxane withviscosities ranging from 1.000-300.000 centistokes. Typically, siliconeoil(s) are present together with fluorinated hydrocarbons in an amountof from 0.1-5 wt%, and preferably from 0.5-2 wt%, and preferably from0.5-2 wt% based on amount of polyketone.

The combination of useful silicone oils and fluorinated hydrocarbonsgives a further improvement in DCOF. It is thought that the silicone oiladheres preferentially to the fluoropolymers and acts as a dispersingagent, thereby reducing coalescence during the compounding step. Abetter dispersion of the fluoropolymer is thought to lead to superiortribological properties, and in some cases to improvements in impactenergy and general overall toughness.

Further, the polymers can contain additives which are known in the art,such as flame retardants, stabilizers, pigments, mold release agents,antioxidants and fillers.

The polymer containing parts can be manufactured by for example aprocess such as injection molding.

It will be clear that an arrangement according to the present inventioncan contain more than two parts which rub against each other.Tribological arrangements in which the set-up according to the presentinvention can advantageously be used, are gears, bearings, ball-bearings, cams, slides, ratchets, pumps, electrical contacts andprostheses.

EXAMPLES 1-4

In each of the examples 1-4, neat polymer was processed to fabricatespur gears. Two different types of polyketone polymer were used: apolyketone homopolymer formed from ethylene and carbon monoxide was usedto fabricate one set of polyketone gears and a polyketone copolymerformed from ethylene, carbon monoxide, and propylene was used tofabricate another. In each example it was found that both sets ofpolyketone gears performed substantially identically. Thus, results forboth sets of polyketone gears are reported as Gear A. A nylon 6,6composition sold under the tradename "ZYTEL 101" by E.I. du Pont deNemours & Co. was used to fabricate Gear B and an acetal copolymer soldunder the tradename "CELON M90" by Hoechst Celanese Corporation was usedto fabricate Gear C.

Spur Gear mold inserts obtained from ABA PGT Inc. of Manchester,Connecticut were modified so that the resulting gears would be of twovarieties: a 33 tooth gear (33T), and a 34 tooth gear (34T). Polyketonepolymers were injection molded without the use of processing aids oradditives. Acetal and nylon gears were injection molded with theinclusion of processing aids and additives as sold with the polymer inits pelletized form. Each polymer used to produce a given size gear wasprocessed through the same mold (e.g., all 34T gears were made from thesame mold, etc.). Results were averaged for all gears of a given type.For example, results reported for weight loss for Gear A is the averageweight loss for 33T and 34T tested as a meshing pair. In each case, thegears were formed to have an involute geometry with tip and root relief.Each of the 33T and 34T gears had a diametral pitch of 12 andtheoretical pitch diameters of 7.19 cm (2.83 inches) and 6.99 cm (2.75inches) respectively.

In each example, each gear was tested without lubrication.

EXAMPLE 1

Gears were affixed to a four-square gear tester having two shafts; adrive shaft connected to a variable speed motor and a torsion barparallel to the drive shaft. One steel gear was affixed to each shaftwhich were then placed in communication with each other so that themotion of the drive shaft was transmitted to the torsion bar. At the endof the shafts opposite the motor, a gear made of polymer was affixed toeach shaft; one a 33T and the other a 34T. The two polymer gears wereplaced in communication with each other. Thus, motion was alsotranslated between the two polymer gears. A torquemeter was placedapproximately midway along the drive shaft to measure and adjust theamount of torque placed on the system.

The four-square gear tester was run at 1200 revolutions per minute (RPM)giving a pitch line velocity of 264 m/min (865 ft/min). Torque was heldconstant throughout each run. Gear life was determined by running thetester under these conditions until torque could no longer betransferred from one polymeric gear to the other. Results are shown inFIG. 1.

This example shows that polyketone gears endure torques for a muchgreater period of time (as measured by cycles of operation) than docommonly used polymeric gears. Further, it was found that gear toothfailure occurred in polyketone gears as a result of mechanical fatiguefailures. Failures in the case of each of the other polymers occurred asa result of wear failures. This shows that, unlike most other polymericgear systems, polyketone gear systems can be stressed to theirmechanical limit without significant loss in tooth dimensions even whenthe gears in direct communication with each other are comprised of thesame polymer.

EXAMPLE 2

This example was conducted as set forth in Example 1 except that thefour-square gear tester was periodically stopped so that gears could bedisassembled and weighed. Material loss was taken as an indication ofgear wear. Further, two different runs were conducted for each gear set:one run at a torque of 4.3 N.m (38 in-lbs) and the other at 11 N.m (95in-lbs). Results of the 4.3 N.m example are shown in FIG. 2a. Results ofthe 11 N.m example are shown in FIG. 2b.

This example shows that polyketone gear systems exhibited a level ofwear far lower than that found in commonly used polymeric gear systems.It was also observed that failure occurred in the case of polyketonegear systems as a result of mechanical fatigue while other polymericgear systems failed because of wear/material loss. That is,nonpolyketone gear systems failed because excessive tooth wear resultedin slippage and the inability to continue transmitting motion and powerwhile the polyketone gear system transmitted motion and power up to thepoint of tooth breakage.

EXAMPLE 3

A noise isolation box was placed around the gears in the four-squaretester used in each of the examples cited above. Two microphones wereplaced inside the box and were connected to "MODEL 2610" amplifierscommercially available from Bruel and Kjaer amplifiers and subsequentlyto a Nakamichi DMP 100 converter to convert analog signals to digitaloutputs. The outputs were then recorded on a Panasonic AG2400 VHSrecorder. The recorded digital outputs were analyzed using a Bruel andKjaer model 2032 FFT analyzer. Two separate analyses were conducted: 1)a narrow band analysis using a 16 Hz bandwidth in the frequency band of0-1, 2800 Hz, and 2) an octave band analysis.

When recordings were made without the actuation of the gears, it wasfound that any noise generated outside of the box with a frequencygreater than about 175 Hz would not affect noise measurements inside thebox. Gear fabrication was the same as that of Example 1. The resultsshown in FIG. 3 are from a run in which 5.7 N.m (50 in-lbs) of torquewere applied.

EXAMPLE 4

A disk was machined from injection molded plaques of neat polyketone(Disk A) and acetal homopolymer (Disk B) sold under the tradename"DELRIN II 500" by E.I. du Pont de Nemours and Co. A stationary pin wasmachined from injection molded tensile specimens of the same materials.The disk and pin were then used in a standard pin-on-disk set-up usingstandard geometries. Surfaces in sliding contact were not altered fromthe as-injected molded state by the machining.

The as-machined geometry of the pin was a square block of dimensions 1cm (0.394 inches) on each edge. The machined disk was rotated at a rateyielding an average surface velocity of 0.25 cm/s (49 ft/min) over theannulus resulting from the pin contacting the disk during the rotation.The pin was pressed against the disk with a force yielding a bearingpressure of 5×10⁶ N/m² (725 lb/in²). Table 1 shows the results obtained.The dynamic coefficient of friction (DCOF) and wear factors weredetermined with the wear factor being the average of both pin and disk.

The lower DCOF of the polyketone is indicative of higher lubricity.This, together with a wear factor two orders of magnitude lower than theacetal homopolymer shows that systems of polyketone means fortransmitting motion and power in which sliding motion predominates aresuitable while means made of other substances are not. Thus, suchpolyketone means include, for example, cams on plates, rotary bearings,and gear systems wherein power and motion is transmitted betweennon-parallel and non-intersecting shafts.

                  TABLE 1    ______________________________________    Polymer  Dynamic Coefficient of Friction                         ##STR1##    ______________________________________    Disc A   .3         .21    Disc B   .4         21    ______________________________________

The wear factor is the volume of material worn off per unit of distancewhich the pin moved along the disc and per unity of force appliedbetween pin and disc.

EXAMPLE 5

The polyacetal-H used in the experiment is Delrin II 500 (DELRIN is atrade mark) which is commercially available from Du Pont. The polyamide6 is ORGAMIDE RMNCD (ORGAMIDE RMNCD is a trade mark) commerciallyavailable from Atochem.

The polyketone can be prepared as follows. A mechanically stirredautoclave with a capacity of 100 1 is charged with 45 kg of methanol and3.5 kg of propene. After the contents of the autoclave is brought to 75° C., a 1:1 carbon monoxide/ethene mixture is blown in until a pressureof 45 bar is reached. Subsequently, a catalyst solution is introducedinto the autoclave comprising 100 ml methanol, 100 ml toluene, 0.75 mmolpalladium acetate, 15 mmol trifluoro acetic acid, and 0.90 mmol 1,3-bisdi(2-methoxyphenyl)phosphino!propane.

During polymerization, the pressure is kept at 45 bar by theintroduction of a 1:1 carbon monoxide/ethene mixture. Polymerization isterminated after 47 hours by cooling the reaction mixture to roomtemperature and releasing the pressure. After the polymer suspension hasbeen withdrawn through an opening in the bottom of the autoclave, theautoclave is flushed with 45 1 of methanol in order to remove polymerremaining behind. The methanol suspensions are combined and filtered.The terpolymer is washed with methanol and dried at 50° C. The yield is5.2 kg of terpolymer with an LVN(60) of 1.88 dl/g, a bulk density of 290kg/m³ and a melting point of 228° C.

Pins and discs were formed and tested in a test set- up as described inISO 7148 of a pin which was pressed onto a disk with a contact pressureof 5 MPa while the pin and disk moved with respect to each other at aspeed of 0.25 m/s. In Tables 2 and 3 the wear has been given in 10⁻¹⁵ m³/Nm.

                  TABLE 2    ______________________________________                         wear     wear   total                         factor   factor of                                         wear    disc       pin       of pin   disc   factor    ______________________________________    polyacetal-H               polyketone                         0.2      0.1    0.3    polyacetal-H               polyamide 6                         0.5      3.8    4.3    ______________________________________

                  TABLE 3    ______________________________________                         wear     wear   total                         factor   factor of                                         wear    disc       pin       of pin   disc   factor    ______________________________________    polyketone polyacetal-H                         2.9      2.8    5.7    polyketone polyamide 6                         0.7      8.3    9.0    ______________________________________

EXAMPLE 6

Pins and discs were tested in accordance with the procedure described inISO 7148. Pins and discs were prepared from a polyketone prepared fromcarbon monoxide, ethene and a minor amount of propene; from polyamide6,6 and from polyoxymethylene. The pins and discs were tested for 20hours at a pressure of 2 MPa and at a velocity of 0.2 m/s. In Table 4the total wear factor of pin and disc has been given in 10¹⁵ m³ /N.m.

                  TABLE 4    ______________________________________                                  pin:             pin       pin        polyoxy-             polyketone                       polyamide 6.6                                  methylene    ______________________________________    disc:      13.4        13.6       11.7    polyketone    disc:      15.9        76.3       4.6    polyamide 6.6    disc:      0.82        1.66       513.7    polyoxy-    methylene    ______________________________________

I claim as my invention:
 1. A tribological arrangement wherein at leasttwo parts, a first part and a second part which are in contact at theirsurfaces, said first part consists essentially of an aliphaticpolyketone while the second part consists essentially of a polymer whichis not an aliphatic polyketone.
 2. The arrangement of claim 1 whereinthe polyketone is a linear alternating polymer which has been preparedfrom carbon monoxide and one or more olefinically unsaturated compounds.3. The arrangement of claim 2 wherein the unsaturated compound isselected from the group consisting of ethene, propene, butene, andcombinations thereof.
 4. The arrangement of claim 1 wherein the polymerwhich is not an aliphatic polyketone is comprised of polyacetal.
 5. Thearrangement of claim 4 wherein the polyacetal is polyoxymethylene. 6.The arrangement of claim 1 further comprising an internal lubricant. 7.The arrangement of claim 2 further comprising an external lubricant. 8.The arrangement of claim 1 comprising a set of gears.
 9. The arrangementof claim 1 comprising a bearing.